The present invention relates to a method of producing high-purity metallic chromium, and more particularly, relates to a method of preparing said mixed material of chromium oxide and aluminum by adding carbon and an easily sulfidable metallic powder thereto in advance,
producing thermite-crude metallic chromium by providing thermite reaction with said mixed material so as to make a solid solution of carbon and easily sulfidable metal, and,
removing residual impurities such as oxygen and sulfur in said crude metallic chromium by heat treating said crude metallic chromium in a vacuum, and a method of producing high-purity metallic chromium is proposed which is advantageous in the field of electronics and as a material for corrosion and heat resistant chromium-containing alloys.
Recently metallic chromium is widely used as a material for semi-conductors, electronic parts, dry plating, etc., and in these fields metallic chromium which is low in gas components such as oxygen and nitrogen, as well as metallic chromium containing low percentages of sulfur are desired.
In the prior art of producing metallic chromium, a method using a mixed raw material of chromium oxide and aluminum is well known wherein the thermite reaction is utilized, which take place because of the reaction heat produced when chromium oxide is reduced by aluminum. Because this technology, so-called "Thermite Method", can raise the purity of the obtained metallic chromium by selecting raw materials whose impurities contents are low, and controlling the reaction speed, and because it can obtain more high-purity metallic chromium of chemically high-quality than electrolyting a Cr.sub.2 (SO.sub.4).sub.3 -solution (so-called "Electrolytic Method", it is the most suitable method for preparing materials used in the field of electronics.
The metallic chromium obtained by the thermite method is, however, inevitably contaminated with oxygen in the form of Al.sub.2 O.sub.3, Cr.sub.2 O.sub.3, etc., as well as with nitrogen in the form of Cr.sub.3 N, etc., and it is also inevitably contaminated with sulfur in spite of carefully selected raw materials such as chromium oxide, etc. used for thermite reaction. And, therefore, gas components such as oxygen, nitrogen and sulfur in the thermite-metallic chromium are generated after completion of thermite reaction. These gas components are not small and are harmful impairing the performance of electronic materials and parts. It is therefore advantageous that these components are present as little as possible.
Conventionally, as a method of reducing these impurities gas components to be as little as possible, such a method as that disclosed in Japanese Patent Laid-Open No. 59-56540 is well known. This technology is a method which comprises adding carbon to the metallic chromium after thermite reaction obtained by thermite method, heating it in the vacuum furnace, thereby reducing oxides present in the metallic chromium and concurrently pyrolizing nitrides and sulfides in order to remove oxygen, nitrogen and sulfur in the thermite-metallic chromium.
And the technology disclosed in Japanese Patent Laid-Open No. 63-282217 is a proposal relating to a method wherein an easily sulfidable metallic powder is added to the thermite-metallic chromium powder, mixed therewith, and heated in vacuum in order to remove sulfur.
The method disclosed in said Japanese Patent Laid-open No. 59-56540 is a technology wherein a reducing agent is added to thermite-metallic chromium and heated together at the time of heat treatment in order to remove oxygen present in the form of oxides in the thermite-metallic chromium by reduction. In the detailed method the thermite-metallic chromium is ground first and then carbon powder is added thereto in order to bring them into contact with each other sufficiently, and, if necessary, an agglomerating agent is added and mixed. The mixture is then molded and the obtained molding is heated in vacuum. In this conventional method, however, it is difficult to mix metallic chromium powder and carbon perfectly homogeneously with each other. After heating, therefore, there were portions where oxygen had been insufficiently removed, and carbon remained sometimes unreacted in metallic chromium products.
In addition thereto, there were disadvantages that contaminants from the grinder, etc. at the time of grinding could lead to contamination, and that impurities heavy metals such as Fe, etc. were inevitably contained. Besides, the fact was that no contaminations from an agglomerating agent or a molding machine could be avoided when using an agglomeration agent.
There was also a disadvantage that sulfur could be removed only insufficiently because sulfur was removed by pyrolysis according to this conventional method.
Moreover, this conventional method was also economically disadvantageous because of the grinding and molding processes thereof, and had to be improved as a matter of course when also taking into consideration that each process had to be performed extremely carefully so that no contamination with impurities might occur.
As a technology which can eliminate the disadvantages of said conventional technology that no sulfur is removable, the method of said Japanese Patent Laid-Open No. 63-282217 was proposed. Similarly to the method disclosed in said Japanese Patent Laid-Open No. 59-56540, this conventional method also comprises adding easily sulfidable metallic powder to ground thermite-metallic chromium and mixing them afterwards, and subsequently treating the mixture with heat, however, this method has the same problems in that thermite-metallic chromium powder can not be mixed with said easily sulfidable metallic powder homogeneously, that sulfur is removed only insufficiently, and that contamination with impurities occurs at the grinding.
It is therefore an object of the present invention to provide a technology of producing high-purity metallic chromium by thermite method which can eliminate the problems of the conventional method effectively.